When fluorescent lamps break, fragments of the glass tube, mercury, and powders from the phosphor coating inside the lamp are scattered about. In places where food is processed or stored, and particularly in lighted display cabinets in supermarkets where stocking clerks and customers handle food in proximity to the cabinet lamps, it is at least highly desirable, and is often required by government regulations, that the cabinet lamps be protected in a way that minimizes the possibility of the lamps being broken and if they do break that the possibility of glass fragments, mercury, and phosphor powders escaping and contaminating the food be eliminated. Fluorescent lamps with a protective assembly are known and widely used for this purpose. Examples of such lamps are those described and shown in U.S. Pat. No. 4,048,537 (Blaisdell et al., 1977), U.S. Pat. No. 4,924,368 (Northrup et al., 1990), U.S. Pat. No. 5,173,637 (Sica, 1992), U.S. Pat. No. 5,536,998 (Sica, 1996) and U.S. Pat. No. 5,729,085 (Sica, 1998). The three Sica patents are owned by the assignee of the present invention.
Store display cabinets for frozen foods present, in addition to the need for minimizing the possibilities of lamp breakage and escape of glass and powders, a requirement for maintaining the optimum light output of the lamp in the low temperature environment; fluorescent lamps designed to operate at ambient room temperature--about 70.degree. F.--provide much less light output at low ambient temperatures than they provide at room temperature. There have, accordingly, been numerous proposals for adapting fluorescent lamps for low-temperature operation, such as those disclosed in the following U.S. Pat. No. 2,135,696 (Baumhauser et al., 1938); U.S. Pat. No. 2,363,109 (Keiffer, 1944); U.S. Pat. No. 2,581,959 (Koehler, 1952); U.S. Pat. No. 3,358,167 (Shanks, 1967); U.S. Pat. No. 3,453,470 (Hammer, 1969); U.S. Pat. No. 3,602,759 (Evans, 1971); U.S. Pat. No. 3,720,826 (Gilmore et al., 1973); and U.S. Pat. No. 4,916,352 (Haim et al., 1990).
In most instances, the prior art protective assemblies for fluorescent lamps include special molded end fittings that fit onto the terminal caps of the lamp and hold a protective sleeve in place on the lamp. In some cases, the end fittings are intentionally designed so that they can be removed from the lamp, thus allowing the protective assembly to be reused as lamps burn out and are replaced. Removable protective assemblies do not fully ensure that glass fragments and phosphor powders are retained in case the lamp breaks. Regardless of whether the end fittings are permanently attached to the lamp or are removable, they are relatively expensive and usually have to be designed for a specific lamp style to ensure a proper fit.
The lamp of the Sica '637 patent (referred to above) provides excellent protection against breakage of the lamp, and if the lamp should break, the protective assembly provides an enclosure that is secure against scattering of glass fragments and phosphor powders. In that regard, a protective tube over the lamp tube is securely joined to the terminal caps of the lamp by collars that both mechanically engage and are adhesively secured to the protective tube and the terminal caps. The lamp of the Sica '637 patent is inexpensive to produce, inasmuch as it uses simple tubular elements and commercially available adhesives.
The Sica '998 patent, which is also referred to above, relates to a protected fluorescent lamp that is constructed to operate in cold environments, such as supermarket freezer cabinets and chests or in outdoor environments in cold weather. It uses a standard "room temperature" (70.degree. F.) fluorescent lamp and has a protective tube held in radially spaced-apart relation to the glass tube of the lamp by a compressible foam spacer ring at each end. The spacing between the glass tube of the lamp and the protective tube provides an air gap between the lamp tube and the protective tube. The air trapped in the air gap provides an insulating layer for restricting heat transfer from the lamp to the ambient air outside the protective tube and for circulation of air for convective distribution of heat along the length of the lamp. The retention of heat in the air gap prevents the lamp from cooling, thereby maintaining the light output at or close to the output of the lamp at room temperature. The protective tube absorbs impacts and minimizes the possibility of the lamp being broken. Should the lamp break, the shrink fit collars, which are joined to the flanges of the lamp terminal caps and the protective tube by an adhesive as well as by mechanically engaging the flanges and the protective tube, maintain the integrity of an enclosure, which is composed of the protective tube, the collars, and the terminal caps, for containment of lamp fragments and phosphor powders.
The foam spacer ring provides several functions. First, it is a spacer for the protective tube from the glass tube of the lamp, in that it establishes the air gap uniformly with respect to the lamp tube at each end. Because the protective tube is substantially rigid, the uniformity of the air gap along the entire length of the lamp is ensured by establishing the gap at each end. Second, the foam spacer rings provide thermal insulation at the ends of the air gap and prevent cold spots from forming where the protective tube ends. Third, the foam spacer rings provide impact-absorbing cushions between the protective tube and the lamp tube, this reducing the possibility of impacts to the tube, especially near the ends, causing the lamp to fracture. The spacer rings are joined to the lamp tube by an adhesive, thus ensuring that they will stay in place, not only when the protective tube is slid over the lamp when the protective assembly is installed on the lamp but throughout the life of the lamp. The fit between the spacer ring and the protective tube is a "snug fit," in order to enhance the insulating function but to avoid stressing the lamp tube and to permit sliding the protective tube over the lamp with the spacer rings already in place. Because the ring is a compressible foam, slight compression is possible without making it difficult to slide the protective tube over the lamp.
The Sica '998 and '637 patents are hereby incorporated into the present document for all purposes.
Lamps with protective assemblies according to the Sica '998 patent, which are commercially available, work very well in the cold temperatures for which they are intended, namely, in a range of temperatures from freezing (32.degree. F.) to -30.degree. F. In the higher part of the range and at temperatures above freezing, those lamps tend to overheat, which reduces their lives, and the end portions of the protective covers may exhibit discoloration over time, which reduces the light output. In some environments, such as cold cabinets for produce, dairy products, some salad dressings, and other products that are kept chilled but somewhat above freezing, protected lamps according to the Sica '998 patent, do not serve well. On the other hand, protected room temperature lamps, such as those of the Sica '637 patent are also unsatisfactory due to reduced light output at temperatures below about 50.degree. F.